In mobile communication devices such as cellular phones, a power amplifier circuit is used for amplifying power of a radio frequency (RF) signal to be transmitted to a base station. In a power amplifier circuit, a heterojunction bipolar transistor (HBT) which exhibits high power-added efficiency and linearity is commonly used as an amplifier element. For example, Japanese Unexamined Patent Application Publication No. 2008-35487 discloses a power amplifier circuit in which a first HBT (Q1) operating as class B to AB and a second HBT (Q2) operating as class C are connected in parallel with each other on the same semiconductor chip.